(1) Field of the Invention
The present invention relates to a stepping motor drive (that is, a driving device of a stepping motor).
(2) Description of the Related Art
So far, for example, an indicating device for use in a vehicle has been known as a device which uses a stepping motor drive. The indicating device indicates measured values measured by various sensors with a pointer. A stepping motor is used to drive the pointer.
A stepping motor drive, which drives the stepping motor, rotates the stepping motor in response to a moving quantity (θ-θ′) which is a difference between a present angle position θ′ of a pointer and a target angle position θ thereof. Thereby, the pointer is moved by the moving quantity (θ-θ′) so as to indicate the target angle position θ. The target angle position θ is updated to an angle datum θi whenever the angle datum θi is inputted, said angle datum θi being computed on the basis of a measured value measured by various sensors.
The stepping motor drive supplies periodic drive signals having phases different from each other so as to periodically change an excitation state of an excitation coil in the stepping motor, so that a magnet rotor surrounded by the excitation coil is allowed to generate rotation torque, thereby rotating the stepping motor.
The indicating device might possibly have a step-out problem in which an actual moving quantity of the pointer is different from the moving quantity (θ-θ′) which is what it should be, due to inputting of the angle datum θi superimposed with a vibration or noise of a vehicle. When such a step-out is repeated, an error takes place between a measured value indicated by the pointer and a measured value measured by the various sensors and therefore, a proper indication cannot be performed.
In order to solve such a problem, the stepping motor is provided with: an abutting piece as a driven member which interlocks with rotation of the stepping motor; and a stopper which mechanically stops the rotation of the stepping motor by abutting against the abutting piece. The stopper is formed in such a manner that the pointer indicates a measured value being zero when the stopper abuts against the abutting piece.
For example, the stepping motor drive rotates the stepping motor so that the abutting piece approaches the stopper whenever a power supply is turned on, so that the abutting piece is forced to abut against the stopper, thereby performing an initialization action in which the pointer is forced to stop at a position of the measured value being zero.
By performing the initialization action, when the present position θ′ is recognized to be the measured value being zero, the pointer also actually indicates the measured value being zero. Accordingly, an error between the measured value indicated by the pointer and the measured value measured by the various sensors can be reset.
However, as for the indicating device described above, it is not known where the abutting piece is positioned upon the initialization, therefore it is necessary to rotate the abutting piece, for example, by 360° in order to make the abutting piece securely abut against the stopper. Therefore, even when the abutting piece abuts against the stopper during the rotation thereof within 360° and the stepping motor cannot rotate further in a direction facing the stopper, such a condition takes place that the excitation state of the excitation coil in the stepping motor continues to periodically change.
On such a condition described above, the stepping motor generates alternately a rotation torque in the direction approaching the stopper and a rotation torque in the direction leaving the stopper while the excitation state of the excitation coil changes by one cycle. Thereby, abutting of the abutting piece against the stopper and recoiling of the abutting piece leaving from the stopper are alternately repeated and thereby, the pointer behaves to recoil from the indicated position of the measured value being zero, causing a problem that the pointer looks unattractive upon the action of initialization.
In order to reduce such a recoil movement of the pointer, there is a method of reducing the rotation torque within a specific phase range upon the action of initialization of the stepping motor (for example, see Japanese Patent Application Laid-Open No. 2004-364403).
FIG. 5 shows an indicating device equipped with a conventional stepping motor drive, in which the method described above is implemented. The indicating device includes a stepping motor 1, a pointer 2 driven by the stepping motor 1, and a driving section 3 controlling rotation of the stepping motor 1. The stepping motor 1 includes: two excitation coils 1a1 and 1a2; a magnet rotor 1b rotating in response to a change in the excitation state of the excitation coils 1a1 and 1a2, the magnet rotor 1b being subjected to magnetization of five N-poles and five S-poles arranged alternately; and a gear 1c which transmits driving force of the magnet rotor 1b to the pointer 2.
The stepping motor 1 further includes: an abutting piece 1d, which is formed on the back side of the gear 1c situated on the side of the pointer 2 and is a driven member that interlocks with rotation of the magnet rotor 1b; and a stopper 1e, which is formed in a receiving case (not shown in the figure) receiving the excitation coils 1a1 and 1a2, the magnet rotor 1b, the gear 1c and the abutting piece 1d, and mechanically stops the rotation of the magnet rotor 1b by abutting against the abutting piece 1d. The stopper 1e is formed in such a manner that the pointer 2 indicates a scale of the measured value being zero on a dial from the above when the stopper 1e abuts against the abutting piece 1d. 
Hereinafter, in this specification, a rotation direction of the stepping motor 1, in which the abutting piece 1d approaches the stopper 1e, is defined as a reverse rotation direction, while a rotation direction of the stepping motor 1, in which the abutting piece 1d leaves the stopper 1e, is defined as a normal rotation direction.
The driving section 3 consists of a microcomputer including: a central processing unit (CPU) 3a, which performs various processing and control according to a predetermined program; a read only memory (ROM) 3b, which stores a program and so on for the CPU 3a; and a random access memory (RAM) 3c, which stores various data and has an area required for the processing of the CPU 3a. 
The excitation coils 1a1 and 1a2 in the stepping motor 1 are connected to the driving section 3. Receiving the periodic drive signals outputted from the driving section 3, the excitation state of each of the excitation coils 1a1 and 1a2 changes, so that a rotation torque is generated in the magnet rotor 1b. An inputting of the power supply is started in the driving section 3, for example, on a timing of turning-on of an ignition switch.
In the following, as for the indicating device having such a construction as described above, a processing sequence of the initialization action performed by the CPU 3a will be explained with reference to a flow chart shown in FIG. 6. When an inputting of the power supply from an on-vehicle battery is carried out, the CPU 3a starts the initialization action. First, a present position θ′ of the pointer 2 stored in the RAM 3c is read out (step S1). The present position θ′ of the pointer 2 of course also corresponds to a present position of the abutting piece 1d. 
The RAM 3c belongs to a nonvolatile-type, in which the contents are maintained therein even in the event of cutting off of the power supply. That is, if the start of the inputting of the power supply is a recover from a condition, in which the inputting of the power supply to the driving section 3 is provisionally cut off, in response to a start of an engine, the present position θ′ of the pointer 2 stored in the RAM 3c corresponds to a position indicated by the pointer 2 before the provisional cutting-off.
Then, the CPU 3a supplies a drive signal B of the reverse rotation direction to the stepping motor 1 by an amount, which corresponds to a difference (−θ′) between the read-out present position θ′ and an initial position 0° (i.e. the stopper position) (step S2). By the processing at the step S2, the abutting piece 1d and the pointer 2 rotate in the reverse rotation direction by an amount of −θ′.
FIG. 7A is an example of a torque control pattern illustrating a relation between a phase φ of a drive signal B and a rotation torque generated in the stepping motor 1 in the indicating device shown in FIG. 5. FIG. 7B is a time chart illustrating a COS current and a SIN current based on the torque control pattern shown in FIG. 7A. As shown in FIGS. 7A and 7B, the driving section 3 supplies such a drive signal B that a rotation torque T generated in the stepping motor 1 always stays constant. At that time, a COS current having a constant amplitude and a SIN current having a phase shifted by 90° with respect to the COS current shown in FIG. 7B flow in the excitation coils 1a1 and 1a2. Upon a normal action, in which a measured quantity on a traveling condition of a vehicle is indicated, the driving section 3 supplies such a drive signal B of the normal rotation direction that the rotation torque T always stays constant. On the other hand, at step S2 upon the initialization action, a drive signal B of the reverse rotation direction shown in FIG. 7A.
When the outputting of the drive signal B is finished, the CPU 3a supplies a drive signal A of the reverse rotation direction to the stepping motor 1 (step S3). By the processing at step S3, the pointer 2 rotates toward the measured value being zero while the abutting piece 1d rotates toward the stopper 1e. 
FIG. 8A is an example of a torque control pattern illustrating a relation between a phase φ of a drive signal A and a rotation torque generated in the stepping motor 1 in the indicating device shown in FIG. 5. FIG. 8B is a time chart illustrating a COS current and a SIN current based on the torque control pattern shown in FIG. 8A. As shown in FIGS. 8A and 8B, the driving section 3 supplies such a drive signal A that with respect to a first rotation torque generated in the stepping motor 1 upon supplying the drive signal A for a phase range R2 (from 0° to 180°) in one electric cycle, a second rotation torque generated in the stepping motor 1 upon supplying the drive signal A for a remaining phase range R1 (from 180° to 360°) in the one electric cycle becomes a half of the first rotation torque. At that time, as shown in FIG. 8B, a COS current and a SIN current, in which the amplitude for the phase range R1 is a half of amplitude for the phase range R2, flow in the excitation coils 1a1 and 1a2.
Thus, if the drive signal A of the reverse rotation direction is supplied to the stepping motor 1 at step S3, the rotation torque generated in the stepping motor 1 can be made small for the phase range R1, in which the rotation torque of the normal rotation direction, that is, the rotation torque of a direction in which the abutting piece 1d recoils from the stopper 1e is generated. Therefore, the speed of reversing from the reverse rotation direction to the normal rotation direction becomes low and therefore, the abutting piece 1d can be prevented from recoiling. On the other hand, a large rotation torque is generated for the phase range R2, in which the rotation torque of the reverse rotation direction, that is, the rotation torque of a direction in which the abutting piece 1d is pressed to the stopper 1e is generated and therefore, the abutting piece 1d can be further prevented from recoiling.
Then, waiting the abutting piece 1d to rotate by 360° (Y at step S4), the CPU 3a halts to supply the drive signal A, so that the processing is finished.
In the method as described above, upon the initialization action when the abutting piece 1d connected with the rotation of the stepping motor is made abut against the stopper 1e, the driving section 3 supplies such a drive signal A that the rotation torque generated in the stepping motor 1 when the drive signal A is supplied for a predetermined phase range R1 of phase angle 360° becomes smaller than the rotation torque generated when the drive signal A is supplied for a remaining phase range R2 of the phase angle 360°, and the magnitude of the rotation torque generated in the stepping motor 1 is controlled for a specific phase range. Thereby, the recoil movement of the abutting piece 1d can be reduced and therefore, the recoil movement of the pointer 2 can be reduced.
However, in the method described above, there is dispersion in a phase angle (i.e. excitation position) at which the inversion of the magnet rotor 1b in the stepping motor 1 takes place due to dispersion in backlash of the gear 1c, dispersion in a forming position of the stopper 1e and dispersion in a magnetization width of a N-pole and a S-pole of the magnet rotor 1b, and a phase angle at which an inversion torque is generated varies depending on an individual motor, causing a problem that there is dispersion in the movement of the pointer 2 upon the initialization action described above.
That is, as shown in FIG. 9, in each case when a motor a is used, when a motor b is used and when a motor c is used as the stepping motor 1 with respect to a start point 180° of the phase range R1, which is set to control a torque upon the recoiling of the abutting piece 1d, there is dispersion in a phase angle (i.e. excitation position) at which the inversion from the reverse rotation direction to the normal rotation direction takes place, and a start position of actual recoiling of the abutting piece 1d formed in each motor is shifted from the starting point 180° of the set phase range R1, causing a problem that there is dispersion in the movement of the pointer 2.